This invention relates to a device for coupling a laser beam from a point source to an optical fiber.
Efficient coupling of a solid state laser beam into a single-mode optical fiber has become an issue of enormous importance. Solid state lasers typically produce infra-red output beams in a one micrometer wave length range with total beam spread angle of about 20-40.degree.. These wide beam spread angles must be reduced to angles typically less than 5.degree. for efficient coupling into single-mode fibers. The use of a single-mode fiber is essential in optical communications to eliminate or at least reduce velocity dispersion in the propogated light signal. If dispersion occurs, the useful bandwidth of the communications signal is greatly restricted, which is highly undesirable.
A variety of systems for efficiently coupling solid state lasers into single-mode fibers have been proposed and tested. Many have proven themselves in practice. However, it is believed that the best coupling coefficient that have been achieved are a maximum of about 50%. Most coupling devices incorporate a single lens, a pair of lenses, or GRIN lenses to focus the laser output beam upon the core of an optical fiber. Magnification from 4X to 7X reduces the lens output aperture angle to practical limits. The lenses typically are small, about 1 mm or less in diameter.
It is difficult to manufacture lenses of this size except in simple forms. Small spheres are generally used in both single and double lens coupler designs. The manufacture of small lenses with concave or aspherical surfaces is difficult and costly. Concave or aspherical lenses are used to correct aberration. Third order spherical aberration is the most important problem to be overcome in achieving higher coupling efficiency particularly when using spherical lenses. All spherical two-lens confocal coupler designs reduce spherical aberration somewhat and provide what is believed to be the best reported efficiencies. However, these efficiencies appear to be relatively low and, thus, there is room for improvement.